STScI Logo

Hubble Space Telescope
Count-rate Non-Linearity

NICMOS Count-rate Dependent Non-linearity

The NICMOS team has determined that NICMOS has a significant count rate dependent non-linearity that also depends on wavelength. This is a different non-linearity from the well-known total count dependent non-linearity. The non-linearity in amounts to 0.05-0.10 mag offset per dex change in incident flux for the shortest wavelength (F090M and F110W), about 0.03 mag/dex at F160W and less than that at longer wavelengths (see figure below). Details about how the non-linearity was quantified can be found in the NICMOS ISR 2006-001 and NICMOS ISR 2006-002 Instrument Science Reports (ISRs).

We have developed a python/pyraf routine called "rnlincor" to correct NICMOS data for the non-linearity. The routine is distributed with STSDAS and stsci_python in the 'hst_calib.nicmos' subpackage. Please note that the first version of this routine distributed in June 2007 with STSDAS 3.7 had an error in SKYVAL correction. Users who used pedsky in default mode before using this routine are recommended to rerun the routine with the new rnlincor version if their sources are close to sky level. Please update to STSDAS 3.8+ ,rnlincor 0.8+ (this version number is printed when you run rnlincor) to correct this error.

Next to running in the pyraf environment, the routine can also be used as a stand-alone command line routine or as part of a pure python script. The routine corrects NICMOS count-rate images assuming the non-linearity follows a power-law behavior. The wavelength dependence of the non-linearity is interpolated between the measured points of de Jong et al. (2006) and Bohlin et al. (2006) if necessary. The STSDAS version of this routine superseeds a previous Beta version that was made available through this web site. Most notably, this new version by default applies the correction in such a way that the photometric zero point is not shifted and therefore the PHOTFLAM and PHOTFNU values written in the header (or maybe even more up-to-date values available here) can still be used.

Full details about the how to use the routine can be found in the NICMOS ISR 2006-003 and in the iraf help pages. The routine uses reference files that can be delivered from the MAST archive with the data. The needed reference files can be found in the header keyword items RNLCORTB and ZPRATTAB. Data retrieved from the archive before Oct 2007 will not have these keyword populated yet. The easiest way to populate these headers with the correct values is to obtain the data from the archive again. The routine can be used on fully reduced (even mosaiced) data, as long as the units are in DN/s, not e-/s, and the sky value has not been subtracted.